1. Field of the Invention
This invention relates to a system for measuring the carrier power to noise power ratio of a TDMA signal transmitted through a wired or wireless transmission path.
2. Description of the Prior Art
A variety of communication methods have been heretofore developed and adopted for actual use as means for the transmission of information through transmission paths. In terms of pattern of transmission, they may be roughly divided into an analog communication method and a digital communication method. In the case of the digital communication methods, the communications are carried out both in a continuous mode and a burst mode.
As one criterion for the evaluation of the signal quality in transmission, the carrier power to noise power ratio (hereinafter referred to as C/N for short) at the receiving terminal is important.
With the conventional method for C/N measurement, since the signal components are dispersed in a frequency band where the modulated signal is included, it is difficult to separate noise from signal and measure C/N with high accuracy. For the signal of this kind, therefore, there is adopted either
(1) a method which effects the measurement of C/N by extracting signal components during the absence of modulation through a filter and further measuring noise power in the band as extracted from its signal components, or
(2) a method which effects the measurement of C/N by measuring noise power while the signal components are turned off, then with the signal turned on, measuring the sum of signal power and noise power, and calculating C/N based on the results of the foregoing measurements.
In the case of the burst mode in which the transmission is effected by a fixed cycle, the aforementioned methods (1) and (2) which inevitably involve use of a filter prove impracticable even if part of the burst signal should be extracted somehow or other. This is because the fixed-cycle burst signal contains a spectral component which depends on the repeating cycle in a frequency band and the signal or noise component to be measured cannot be separated owing to the obstacle by the spectral components.
Further, the aforementioned method (2) has the disadvantage that it fails to provide accurate measurement when the amplifier inserted in the transmission path operates in the nonlinear region.